There isn't a way to do this natively.

Some of the Service Graph Connectors would use script operations to do this, whereby they read a system property to know whether to map a value or not.

If you're building a solution that you're planning to put on the ServiceNow Store, you may want to speak with the store app review team for architectural advise. You may be over-complicating things with the new of simplifying the customers ability to adopt your ETL definition